Optimize is-check to instanceof when it eliminates an interceptor

pkg/compiler/lib/src/ssa/codegen_helpers.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of ssa;
 
 /**
  * Replaces some instructions with specialized versions to make codegen easier.
  * Caches codegen information on nodes.
  */
@@ skipping 366 matching lines @@
       }
     }
   }
 
   void visitInstruction(HInstruction instruction) {
     // A code motion invariant instruction is dealt before visiting it.
     assert(!instruction.isCodeMotionInvariant());
     analyzeInputs(instruction, 0);
   }
 
-  // The codegen might use the input multiple times, so it must not be
-  // set generate at use site.
-  void visitIs(HIs instruction) {}
+  void visitIs(HIs instruction) {
+    // In the general case the input might be used multple multiple times, so it
+    // must not be set generate at use site.  If the code will generate
+    // 'instanceof' then we can generate at use site.
+    if (instruction.useInstanceOf) {
+      analyzeInputs(instruction, 0);
+    }
+  }
 
   // A bounds check method must not have its first input generated at use site,
   // because it's using it twice.
   void visitBoundsCheck(HBoundsCheck instruction) {
     analyzeInputs(instruction, 1);
   }
 
   // An identity operation must only have its inputs generated at use site if
   // does not require an expression with multiple uses (because of null /
   // undefined).
@@ skipping 325 matching lines @@
     }
 
     // If [thenInput] is defined in the first predecessor, then it is only used
     // by [phi] and can be generated at use site.
     if (identical(thenInput.block, end.predecessors[0])) {
       assert(thenInput.usedBy.length == 1);
       markAsGenerateAtUseSite(thenInput);
     }
   }
 }